DRIVES AND CONTROLS 



213 



Work Load Applied as Motor Speed Increases 



Fan — *\ 



Moior^ 



Time 



When the motor is directly connected to the driven machine, and the nature of the machine load 

 is such that it increases as the machine speed increases from no load at rest to full load at full speed, 

 as in fans, blowers, and centrifugal pumps, the motor is required to deliver an accelerating torque 

 that can accelerate the increasing load plus the torque required to accelerate the revolving masses. 



At the instant of starting, the inertia and holding torque of the machine may be small enough to 

 be negligible. However, this fact should not be taken for granted, since dry bearings, cold lubricants, 

 deflected shafting, and sprung parts are factors that may set up considerable resistance to starting. 



After the machine has begun revolving, at any instant the rate at which the machine accelerates 

 will depend upon the relation between the motor accelerating-torque versus the WR"^ of the moving 

 machine parts, plus frictional resistance, plus the load that is on the machine at that instant. 



The running torque required of the motor after coming up to speed is mainly determined by the 

 useful work done and the efficiency of the machine. 



Work Load Applied on the Motor When Starting 



Cable , 

 drum I 



^Z2ZZ. 



'>//////?J7}^^/^^?/y))/M ■ 



v/y7777)A 



Si. 



Time 



With the motor connected directly to a machine upon which a heavy work load must be encount- 

 ered at instant of starting, such as in compressors and piston pumps without unloading devices, lifts, 

 and hoists, the torque required to start and to accelerate may be many times greater than that needed 

 to keep the machine in motion after the desired running speed has been reached. 



Especially is this so when the mass of the machine parts is large and their radii of gyration is 

 great. The motor may be able to deliver enough starting torque to turn the loaded machine over 

 slowly, but if the motor is not capable of dehvering sufficient accelerating torque to bring the machine 

 and load up to speed in a short time, heating will probably occur. 



When the motor has to start and stop frequently under full load, the length of time of motor 

 operation as compared with the idle time in the work or duty cycle is an important consideration that 

 governs the generation and dissipation of heat. 



